1. Field of the Invention
This invention relates to novel substituted 2-amino-9-alkylpurines which selectively bind to GABAa receptors. This invention also relates to pharmaceutical compositions comprising such compounds. It further relates to the use of such compounds in treating anxiety, depression, Down""s Syndrome, sleep and seizure disorders, and overdoses of benzodiazepine-type drugs, and enhancing memory. The interaction of certain substituted 2-amino-9-alkylpurines of the invention with a GABA binding site, the benzodiazepine (BDZ) receptor, is described. This interaction results in the pharmacological activities of these compounds.
2. Description of the Related Art
xcex3-Aminobutyric acid (GABA) is regarded as one of the major inhibitory amino acid transmitters in the mammalian brain. Over 40 years have elapsed since its presence in the brain was demonstrated (Roberts and Frankel, J. Biol. Chem 187: 55-63, 1950; Udenfriend, J. Biol. Chem. 187: 65-69, 1950). Since that time, an enormous effort has been devoted to implicating GABA in the etiology of seizure disorders, sleep, anxiety, and cognition (Tallman and Gallager, Ann. Rev. Neuroscience 8: 21-44, 1985). Widely, although unequally, distributed through the mammalian brain, GABA is said to be a transmitter at approximately 30% of the synapses in the brain. GABA mediates many of its actions through a complex of proteins localized both on cell bodies and nerve endings; these are called GABAa receptors. Postsynaptic responses to GABA are mediated through alterations in chloride conductance that generally, although not invariably, lead to hyperpolarization of the cell. Drugs that interact at the GABAa receptor can possess a spectrum of pharmacological activities depending on their abilities to modify the actions of GABA.
The 1,4-benzodiazepines, such as diazepam, continue to be among the most widely used drugs in the world as anxiolytics, sedative-hypnotics, muscle relaxants, and anticonvulsants. A number of these compounds are extremely potent drugs; such potency indicates a site of action with a high affinity and specificity for individual receptors. Early electrophysiological studies indicated that a major action of benzodiazepines was enhancement of GABAergic inhibition. Presently, those compounds possessing activity similar to the benzodiazepines are called agonists. Compounds possessing activity opposite to benzodiazepines are called inverse agonists, and the compounds blocking both types of activity have been termed antagonists.
The GABAa receptor subunits have been cloned from bovine and human cDNA libraries (Schoenfield et al., 1988; Duman et al., 1989). A number of distinct cDNAs were identified as subunits of the GABAa receptor complex by cloning and expression. These are categorized into xcex1, xcex2, xcex3, xcex4, xcex5, and provide a molecular basis for the GABAa receptor heterogeneity and distinctive regional pharmacology (Shivvers et al., 1980; Levitan et al., 1989). The xcex3 subunit appears to enable drugs like benzodiazepines to modify the GABA responses (Pritchett et al., 1989). The presence of low Hill coefficients in the binding of ligands to the GABAa receptor indicates unique profiles of subtype specific pharmacological action.
With the discovery of the xe2x80x9creceptorxe2x80x9d for the benzodiazepines and the subsequent definition of the nature of the interaction between GABA and the benzodiazepines, it appears that the behaviorally important interactions of the benzodiazepines with different neurotransmitter systems are due in a large part to the enhanced ability of GABA itself to modify these systems. Each modified system, in turn, may be associated with the expression of a behavior. Depending on the mode of interaction, these compounds are capable of producing a spectrum of activities (either sedative, anxiolytic, and anticonvulsant, or wakefulness, seizures, and anxiety).
Various compounds have been prepared as benzodiazepine agonists and antagonists. For example J. Med. Chem, 1989, 32, 1020 and J. Med. Chem, 1990, 33, 196 disclose 6-substituted purines.
These compounds however lack any substitution at C-2. Substituted 2-amino-9-alkylpurines have been reported in the literature to be potential antiviral agents; see, e.g., Chem. Pharm. Bull, 1988, 36 1283.
J. Med. Chem., 1987, 30, 109 discloses yet other purines as an inhibitor of DNA polymerase.
Also, Published International Application WO 9720843-A1 discloses purine derivatives as antineoplastic agents. cl SUMMARY OF THE INVENTION
This invention provides novel compounds of Formula I which interact with a GABAa binding site, the benzodiazepine receptor.
The invention provides pharmaceutical compositions comprising the compounds of Formula I. The invention also provides compounds useful in the diagnosis and treatment of anxiety, depression, sleep and seizure disorders, Down""s Syndrome, overdose with benzodiazepine drugs, and for the enhancement of memory. Accordingly, a broad embodiment of the invention is directed to compounds of general Formula I: 
W is oxygen or sulfur;
X is lower alkyl
optionally mono-, di- or trisubstituted with C2-C6 alkenyl, C2-C6 alkynyl, lower alkoxy, halogen, hydroxy, trifluoromethyl, aryl, heteroaryl, C3-C7 cycloalkyl, tetrahydrofuranyl, CO2R where R is hydrogen or lower alkyl, or
aryl or heteroaryl, each of which is optionally substituted with halogen or lower alkoxy; or
X is aryl or heteroaryl
each of which is optionally substituted with up to three groups selected from halogen and lower alkoxy;
Y is hydrogen or lower alkyl;
Z is lower alkyl optionally substituted with alkenyl, alkynyl, aryl or hydroxy; and
T is aryl(C1-C6)alkyl, aryl, or heteroaryl each of which is optionally substituted on the aryl with up to 5 groups selected from halogen, lower alkyl, lower alkoxy, or C1-C6 alkoxy(C1-C6)alkyl.
The compounds are highly selective agonists, antagonists, or inverse agonists for GABAa brain receptors, or prodrugs of agonists, antagonists, or inverse agonists for GABAa brain receptors. Thus these compounds are useful in the diagnosis and treatment of anxiety, sleep and seizure disorders, depression, Down""s Syndrome, overdose with benzodiazepine drugs, and for the enhancement of memory.
The novel compounds encompassed by the instant invention can be described by the general Formula I set forth above or the pharmaceutically acceptable non-toxic salts thereof.
Preferred compounds of Formula I are those where Y is hydrogen and Z represents a straight or branched chain. lower alkyl having 1-6 carbon atoms.
In addition, the present invention encompasses compounds of Formulas IIa and IIb: 
wherein
W, X, Y, and Z are as defined above for Formula I and
Ra and Rb are the same or different and represent hydrogen, lower alkyl, halogen, or ORxe2x80x2 where Rxe2x80x2 is lower alkyl or lower alkoxy, or
Ra and Rb together with the carbon atom to which they are attached form a 5, 6, or 7 membered carbocyclic ring up to two of which members are optionally hetero atoms selected from oxygen, sulfur and nitrogen.
Preferred compounds of Formula IIa and IIb are where Y is hydrogen and Z is lower alkyl.
More preferred compounds of Formula IIa are where Y is hydrogen, Z is lower alkyl, Ra is hydrogen, lower alkyl or lower alxoxy and Rb is hydrogen, halogen, lower alkyl, or lower alkoxy.
More preferred compounds of Formula IIb are where Y is hydrogen, Z is lower alkyl, Ra is hydrogen, and Rb is halogen or lower alkoxy.
In addition, the present invention encompasses compounds of Formula III: 
wherein
W, X, Y, and Z are as defined above for Formula I, m is 1, 2, or 3, and
Ra is lower alkyl, lower alkoxy, or halogen.
Preferred compounds of Formula III are where Y is hydrogen and Z is lower alkyl.
More preferred compounds of Formula III are where Ra is lower alkoxy.
Most preferred compounds of Formula III are where Ra is 4-methoxy.
Preferred compounds of the invention are encompassed by the following formulae: 
wherein
W, X, Y, and Z are as defined above for Formula I and
Rc is hydrogen, halogen, lower alkyl, ORxe2x80x3 where Rxe2x80x3 is lower alkyl optionally substituted with lower alkoxy.
More preferred compounds of Formula IVb are where Rc is hydrogen, methoxy, ethoxy, methyl, or methoxyethoxy.
More preferred compounds of Formula IVa are where Rc is hydrogen, methyl, fluorine, chlorine, or lower alkoxy.
Other preferred compounds of the invention are encompassed by the formulas VIa and Vb: 
wherein
W, X, Y, and Z are as defined above for Formula I and
Rd and Re are the same or different and represent hydrogen, lower alkyl, lower alkoxy, or halogen.
More preferred compounds of Formula Va are where Rd is hydrogen, methyl, methoxy, or isopropoxy and Re is methyl, methoxy or fluoro.
More preferred compounds of Formula Vb are also where Rd is methoxy and Re is methoxy or fluoro.
Other preferred compounds of the invention are encompassed by the formulas VIa and VIb: 
wherein
W, X, Y, and Z are as defined above for Formula I and
A and B are the same or different and represent CH, CH2, O, N, NH, or S.
More preferred compounds of Formula VIa are where A and B are both both O.
More preferred compounds of Formula VIb are where both A and B are O or where A is N and B is S.
Further preferred compounds of the invention are encompassed by the formula VII: 
wherein
T, W, Y, and Z are as defined above for Formula I and
R1 is lower alkyl optionally mono- or disubstituted with lower alkoxy, hydroxy or halogen, or
R1 is hydrogen, lower alkoxy, alkenyl, alkynyl, trifluoromethyl, C3-C7 cycloalkyl, aryl, heteroaryl, tetrahydrofuranyl, or CO2Rxe2x80x2 where Rxe2x80x2 is hydrogen or lower alkyl.
More preferred compounds of Formula VII are where Y is hydrogen, Z is lower alkyl, R1 is hydrogen, lower alkyl, alkenyl, phenyl, pyridyl, or thienyl; and T is (un)substituted phenyl, (un)substituted 2- and 3-pyridyl, quinolinyl, or benzothiazolyl.
Still other preferred compounds of the invention are encompassed by formula VIII: 
wherein
T, W, Y, and Z are as defined above for Formula I and
R2 is aryl or heteroaryl optionally substituted independently with halogen, lower alkyl, or lower alkoxy.
More preferred compounds of Formula VIII are where Y is hydrogen, Z is lower alkyl, and R2is phenyl or pyridyl optionally substituted independently with lower alkoxy or halogen.
By xe2x80x9calkylxe2x80x9d, xe2x80x9clower alkylxe2x80x9d, and xe2x80x9cC1-C6 alkylxe2x80x9d in the present invention is meant straight or branched chain alkyl groups having 1-6 carbon atoms, such as, methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, 2-pentyl, isopentyl, neopentyl, hexyl, 2-hexyl, 3-hexyl, and 3-methylpentyl.
By xe2x80x9calkenylxe2x80x9d and xe2x80x9cC2-C6 alkenylxe2x80x9d in the present invention is meant straight or branched chain alkyl groups having 1-6 carbon atoms and containing at least one double bond, such as, 1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, and 2-methyl-1-propenyl.
By xe2x80x9calkynylxe2x80x9d or xe2x80x9cC2-C6 alkynylxe2x80x9d in the present invention is meant straight or branched chain alkyl groups having 1-6 carbon atoms and containing at least one terminal triple bond, such as, 1-propynyl, 1-butynyl, and 1-pentynyl.
By xe2x80x9calkoxyxe2x80x9d, xe2x80x9clower alkoxyxe2x80x9d, and xe2x80x9cC1-C6 alkoxyxe2x80x9d in the present invention is meant straight or branched chain alkoxy groups having 1-6 carbon atoms, such as, for example, methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, sec-butoxy, tert-butoxy, pentoxy, 2-pentyl, isopentoxy, neopentoxy, hexoxy, 2-hexoxy, 3-hexoxy, and 3-methylpentoxy.
By the term xe2x80x9chalogenxe2x80x9d in the present invention is meant fluorine, bromine, chlorine, and iodine.
By heteroaryl is meant one or more aromatic ring systems of 5-, 6-, or 7-membered rings containing at least one and up to four heteroatoms selected from nitrogen, oxygen, or sulfur. Such heteroaryl groups include, for example, thienyl, furanyl, thiazolyl, imidazolyl, (is)oxazolyl, pyridyl, pyrimidinyl, (iso)quinolinyl, napthyridinyl, benzimidazolyl, benzoxazolyl. Preferred heteroaryls at T or X or both are thiazolyl and pyridyl.
By aryl is meant an aromatic carbocyclic group having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple condensed rings in which at least one is aromatic, (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl), which is optionally mono-, di-, or trisubstituted with, e.g., halogen, lower alkyl, lower alkoxy, lower alkylthio, trifluoromethyl, lower acyloxy, aryl, heteroaryl, and hydroxy. A preferred aryl is phenyl. A preferred phenyl group is unsubstituted or monosubstituted with lower alkoxy or halogen.
The following numbering system is used to identify positions on the purine ring system of the compounds of the invention: 
Representative compounds of the invention are shown below in Table 1.
Representative compounds of the present invention, which are encompassed by Formula I, include, but are not limited to the compounds in Table I and their pharmaceutically acceptable salts. In addition, if the compound of the invention is obtained as an acid addition salt, the free base can be obtained by basifying a solution of the acid salt. Conversely, if the product is a free base, an addition salt, particularly a pharmaceutically acceptable addition salt, may be produced by dissolving the free base in a suitable organic solvent and treating the solution with an acid, in accordance with conventional procedures for preparing acid addition salts from base compounds.
Non-toxic pharmaceutically acceptable salts include salts of acids such as hydrochloric, phosphoric, hydrobromic, sulfuric, sulfinic, formic, toluenesulfonic, methanesulfonic, nitric, benzoic, citric, tartaric, maleic, hydriodic, alkanoic such as acetic, HOOxe2x80x94(CH2)nxe2x80x94CO2H where n is 0-4, and the like. Those skilled in the art will recognize a wide variety of non-toxic pharmaceutically acceptable addition salts.
The present invention also encompasses the acylated prodrugs of the compounds of Formula I. Those skilled in the art will recognize various synthetic methodologies which may be employed to prepare non-toxic pharmaceutically acceptable addition salts and acylated prodrugs of the compounds encompassed by Formula I.
The compounds of Formula I and their salts are suitable for the diagnosis and treatment of anxiety, depression, Down Syndrome, sleep, cognitive and seizure disorders, and overdose with benzodiazepine drugs and for enhancement of alertness, both in human and non-human animals and domestic pets or companion animals, especially dogs and cats and farm animals such as sheep, swine and cattle.
The compounds of general Formula I may be administered orally, topically, parenterally, by inhalation or spray, or rectally in unit dosage formulations containing conventional non-toxic pharmaceutically acceptable carriers, adjuvants, and vehicles. The term parenteral as used herein includes subcutaneous injections, intravenous, intramuscular, intrasternal injection or infusion techniques. In addition there is provided a pharmaceutical formulation comprising a compound of general Formula I and a pharmaceutically acceptable carrier. One or more compounds of general Formula I may be present in association with one or more non-toxic pharmaceutically acceptable carriers and/or diluents and/or adjuvants and if desired other active ingredients. The pharmaceutical compositions containing compounds of general Formula I may be in a form suitable for oral use, for example, as tablets, troches, lozenges, aqueous or oily suspensions, dispersible powders or granules, emulsions, hard or soft capsules, or syrups or elixirs.
Compositions intended for oral use may be prepared by any method known to the art for the manufacture of pharmaceutical compositions and such compositions may contain one or more agents selected from the group consisting of sweetening agents, flavoring agents, coloring agents, and preserving agents in order to provide pharmaceutically elegant and palatable preparations. Tablets contain the active ingredient in admixture with non-toxic pharmaceutically acceptable excipients which are suitable for the manufacture of tablets. These excipients may be, for example, inert diluents, such as calcium carbonate, sodium carbonate, lactose, calcium phosphate or sodium phosphate; granulating and disintegrating agents, for example, cornstarch or align acid; binding agents, for example starch, gelatin, or acacia; and lubricating agents, for example, magnesium stearate, stearic acid, or talc. The tablets may be uncoated or they may be coated by known techniques to delay disintegration and absorption in the gastrointestinal tract and thereby provide a sustained action over a longer period. For example, a time delay material such as glyceryl monostearate or glyceryl distaerate may be employed.
Formulations for oral use may also be presented as hard gelatin capsules wherein the active ingredient is mixed with an inert solid diluent, for example, calcium carbonate, calcium phosphate, or kaolin, or as soft gelatin capsules wherein the active ingredient is mixed with water or an oil medium, for example peanut oil, liquid paraffin, or olive oil.
Aqueous suspensions contain the active materials with excipients suitable for the manufacture of aqueous suspensions. Such excipients are suspending agents, for example, sodium cerboxymethylcellulose, methylcellulose, hydropropylmethylcellulose, sodium alginate, polyvinylpyrrolidone, gum tragacanth, and gum acacia; dispersing or wetting agents may be a naturally occurring phosphatide, for example, lecithin, or condensation products of an alkylene oxide with fatty acids, for example polyoxyethylene stearate, or condensation products of ethylene oxide with long chain aliphatic alcohols. for example heptadecaethyleneoxycetanol, or condensation products of ethylene oxide with partial esters derived from fatty acids and a hexitol such as polyoxyethylene sorbitol monooleate, or condensation products of ethylene oxide with partial esters derived from fatty acids and hexitol anhydrides, for example polyethylene sorbitanmonooleate. The aqueous suspensions may also contain one or more preservatives, for example, ethyl or n-propyl p-hydroxybenzoate, one or more coloring agents, one or more flavoring agents, and one or more sweetening agents, such as sucrose or saccharin.
Oily suspensions may be formulated by suspending the active ingredient in a vegetable oil, for example arachis oil, olive oil, sesame oil, or coconut oil, or in a mineral oil such as liquid paraffin. The oily suspensions may contain a thickening agent, for example beeswax, hard paraffin, or cetyl alcohol. Sweetening agents such as those set forth above, and flavoring agents may be added to provide palatable oral preparations. These compositions may be preserved by the addition of an anti-oxidant such as ascorbic acid.
Dispersible powders and granules suitable for preparation of an aqueous suspension by the addition of water provide the active ingredient in admixture with a dispersing or wetting agent, suspending agent, and one or more preservatives. Suitable dispersing or wetting agents and suspending agents are exemplified by those already mentioned above. Additional excipients, for example sweetening, flavoring, and coloring agents, may also be present.
Pharmaceutical compositions of the invention may also be in the form of oil-in-water emulsions. The oily phase may be a vegetable oil, for example olive oil or arachis oil, or a mineral oil, for example liquid paraffin, or mixtures of these. Suitable emulsifying agents may be naturally occurring gums, for example gum acacia or gum tragacanth, naturally occurring phosphatides, for example soy bean, lecithin, and esters or partial esters derived from fatty acids and hexitol, anhydrides, for example sorbitan monoleate, and condensation products of the said partial esters with ethylene oxide, for example polyoxyethylene sorbitan momoleate. The emulsions may also contain sweetening and flavoring agents.
Syrups and elixirs may be formulated with sweetening agents, for example gltcerol, propylene glycol, sorbitol, or sucrose. Such formulations may also contain a demulcent, a preservative, and flavoring and coloring agents. The pharmaceutical compositions may be in the form of a sterile injectable aqueous or oleaginous suspension. This suspension may be formulated according to the known art using those suitable dispersing or wetting agents and suspending agents which have been mentioned above. The sterile injectable preparation may also be a sterile injectable solution or suspension in a non-toxic parentally acceptable diluent or solvent, for example as a solution in 1,3-butanediol. Among the acceptable vehicles and solvents that may be employed are water, Ringer""s solution, and isotonic sodium chloride solution. In addition, sterile, fixed oils are conventionally employed as a solvent or suspending medium. For this purpose any bland fixed oil may be employed including synthetic mono or digylcerides. In addition, fatty acids such as oleic acid find use in the preparation of injecatables.
The compounds of general Formula I may also be administered in the form of suppositories for rectal administration of the drug. These compositions can be prepared by mixing the drug with a suitable non-irritating excipient which is solid at ordinary temperatures but liquid at rectal temperature and will therefore melt in the rectum to release the drug. Such materials are cocoa butter and polyethylene glycols.
Compounds of general Formula I may be administered parenterally in a sterile medium. The drug, depending on the vehicle and concentration used, can either be suspended or dissolved in the vehicle. Advantageously, adjuvants such as local anesthetics, preservatives, and buffering agents can be dissolved in the vehicle.
Dosage levels of the order of from about 0.1 mg to about 140 mg per kilogram of body weight per day are useful in the treatment of the above-indicated conditions (about 0.5 mg to about 7 g per patient per day). The amount of active ingredient that may be combined with the carrier materials to produce a single dosage form will vary depending upon the host treated and the particular mode of administration. Dosage unit forms will generally contain between from about 1 mg to about 500 mg of an active ingredient.
It will be understood, however, that the specific dose level for any particular patient will depend upon a variety of factors including the activity of the specific compound employed, the patient""s age, body weight, general health, sex, and diet, and the time of administration, route of administration, rate of excretion, drug combination, and the severity of the particular disease undergoing therapy.
For administration to non-human animals, the composition may also be added to the animal feed or drinking water. It will be convenient to formulate these animal feed and drinking water compositions with a mullet-dose of the drug so that the animal takes in an appropriate quantity of the composition along with its diet. It will also be convenient to present the composition as a premix for addition to the feed or drinking water.
An illustration of the preparation of compounds of the present invention is given in Scheme I. In Scheme I, the groups T, W, X, and Z are as defined in general Formula I. 
Those having skill in the art will recognize that the starting materials may be varied and additional steps employed to produce compounds encompassed by the present invention, as demonstrated by the following examples.
The starting material 2-Mercapto-6-hydroxypurine may be prepared by reacting 4,5-diamino-2-mercapto-6-hydroxypyrimidine with formamide essentially according to the procedures described in J. Am. Chem. Soc., 76: 5633 (Beaman et al., 1954).
As shown in Scheme I, 2-Bromohypoxanthine can be prepared from 2-mercapto-6-hyroxypyrimidine essentially according to the procedures found in J. Org. Chem., 27: 986 (Beaman, 1962). As further shown in Scheme I, the bromide may be displaced with an appropriate amine to afford the respective amino-substituted guanine. The resulting guanine can then be converted to a chloropurine by reaction with an inorganic acid halide, such as, for example, thionyl chloride. The purine may then be N-alkylated at the 9-position with an appropriate alkyl halide in the presence of a base such as potassium carbonate. The chloride may then be converted to the desired alkoxide or thio ether from the respective alcohol or thiol in the presence of a strong base.
The disclosures in this application of all articles and references, including patents, are incorporated herein by reference.
The invention is illustrated further by the following examples which are not to be construed as limiting the invention in scope or spirit to the specific procedures described in them.
The starting materials and various intermediates may be obtained from commercial sources, prepared from commercially available organic compounds, or prepared using well known synthetic methods.